PCR Tube Magnetic Racks

I *loved* the racks designed by destroyer2012 - way better than racks that require gluing the magnets, but I felt the need for some modifications:

# 4-Tube PCR Magnet Rack
This is only slightly modified from https://www.thingiverse.com/thing:1132367 - in the original the spaces in the back of the rack are closed, so you can't see in the PCR tubes there. I opened up the back so the tubes there are as visible in the front. It is 4 tubes wide, so it holds 8 tubes total.

Magnets are the sa